The present invention relates to a vehicle headlamp and, more particularly, a vehicle headlamp having a front lens particularly made of synthetic resin.
The conventional front lens used in vehicle headlamps is made by press-molding a mass of molten glass. A front lens made of synthetic resin has been developed because it can be light-weight and because a plurality of prisms can be easily formed thereon to disperse the light from a filament.
However, where a front lens of synthetic resin is employed, and the lamp filaments are positioned between the focal point of the reflector and the lens; conditions of high temperature or high terminal voltage will cause enough infrared radiation to impinge on the upper part of the lens to soften the lens. This will be described in more detail referring to the conventional headlamp shown in FIGS. 1 and 2. FIG. 1 is a front view showing the conventional headlamp and FIG. 2 is a sectional view taken along the line I--I in FIG. 1.
As shown in FIG. 2, a synthetic resin lens 1 is attached to the front end of a reflector 2. The reflector 2 has a reflecting surface 3 which is made substantially parabolic; to the reflecting surface 3 is fixed a lamp holder 4 for attaching a lamp 6 to the reflector 2. The lamp 6 attached to the lamp holder 4 by means of a set spring 5 includes a main filament 8, a sub-filament 9 and a light shielding cap 10 in a glass envelope 7. Said light shielding cap 10 serves to cover about the lower half of said sub-filament 9. The main filament 8 is nearly positioned at the focal point (f) of said reflector 2, while the sub-filament 9 is positioned ahead the focal point (f). Numeral 11 represents a light shielding coating applied to the front end of the glass envelope 7.
Light radiated from the main filament 8 is reflected by the reflecting surface 3 substantially parallel to an optical axis (X) or diffused. On the contary, light (L) radiated from the sub-filament 9 is reflected by the reflecting surface 3 to be focussed. This is because the sub-filament 9 is positioned ahead the focal point (f).
Vehicles are used under various conditions (on hot deserts, for example). The headlamp of a vehicle which runs on this desert area is under high temperature atmosphere. It is therefore necessary to test the headlamp to see if the headlamp can be used at high temperatures (80.degree. C., for example). When the headlamp exposed as described above is left turned on at air temperatures of 80.degree. C., air inside the headlamp becomes hot due to the hot air surrounding the, and due to headlamp, heat focussed by operation of the lamp 6 itself while on, particularly at the upper portion thereof. As a result, heat is concentrated onto an area (A) at the upper portion of the lens 1 as shown in FIG. 1, so that the lens 1 is softened there.
This softening is caused even under normal temperatures when terminal voltage becomes high. When terminal voltage becomes high, light radiated from the sub-filament 9 becomes intense, and much heat is radiated. Therefore, reflected light (L) and radiated heat combines with the hot air inside the headlamp to soften the area (A).